In the field of rail vehicles, it is known that moving steps can be used to make it easier for passengers to board from a platform and to alight onto a platform, it being possible for such steps to have strokes controlled automatically by means of telemeters in order to optimize the final resulting gap between the step and the platform. Such apparatus is, for example, disclosed in Document U.S. Pat. No. 6,341,563 which describes a system for controlling a moving step relative to a platform. In that system, two telemeters are used to measure the distance between the moving step and the platform by means of a triangulation method, each telemeter being made up of an ultrasound transmitter and detector. Unfortunately, locating the platform by using a location method suffers from the drawback of not enabling the edge coping of the platform to be located very accurately, and of further requiring the two ultrasonic telemeters to be far enough away from the platform and far enough apart to give a result that can be used. As shown in FIG. 1, the triangulation method makes it possible, by means of two ultrasonic telemeters T carried by the vehicle to locate a point P′ corresponding to the point of intersection between two circles representing distance to the obstacle as measured by each of the telemeters. As shown in FIG. 1, the point P′ obtained by the triangulation method gives an acceptable approximation of the point P corresponding to the edge coping of the platform when the two ultrasonic telemeters are far enough apart. However, when the two telemeters T are close together, as shown in FIG. 2, the two circles representing distance to the obstacle as measured by the telemeters cannot intersect, and the triangulation method no longer makes it possible to measure the position of the point P′.